It is known to use X-rays to inspect drink bottles and luggage which are moved past an imaging apparatus onto a transport device. X-ray image intensifiers or converters with a downstream CCD camera are used as imaging systems and the recorded image relayed to an evaluation system. By using a surface sensor in the X-ray image intensifier it is possible to greatly reduce both the radiation energy and also the power input of the X-ray pulse. However, due to the movement of the X-rayed objects, the contours of the images lack sharpness.
If other sensors are used, e.g. line sensors such as a plurality of photomultiplier tubes arranged in a row with the scintillator crystals, all of the energy must be made available continuously even when no object is in the beam path to be inspected. As a result, high radiation energies are released, and high electric outputs are required. Expensive screening and security measures are therefore required for the high connected loads of the equipment and radiation protection.
X-ray units for the generation of brief X-ray pulses are known from DE-C-32 16 733, U.S. Pat. No. 4,947,415 and WO 94/23552. These units serve to generate extremely brief X-ray pulses lasting a few nanoseconds. Specially developed capacitors are used to generate the high-voltage pulse, in order to be able to transmit the high-voltage energy to the anode within an extremely brief pulse duration.
An X-ray unit with an electron field emission cathode is known from WO 02/31857, with which X-ray pulses of different energies can be generated by focusing the electron beam on different anode materials.
An X-ray beam generator for the generation of X-ray pulses is known from EP-A-1 158 842, the high voltage being applied continuously to the anode and the grid voltage being controlled according to the cathode current such that during the period in which no X-ray beams are to be generated, no electrons reach the anode. The pulse duration is also controlled by means of the grid voltage. It is thereby made possible to generate a stable X-ray pulse.